The rotor of a wind turbine, as it rotates, may cast an intermittent shadow on the adjacent ground area. Residents living in this area often perceive such intermittent shadow as a nuisance. This problem might make it difficult to erect or operate wind turbines close to objects that are “sensitive” to intermittent shadow, such as occupied houses.
In this context, it was proposed in DE 199 28 048 A1 that a wind turbine be shut down when a shadow-sensitive object (e.g. a house) close to the wind turbine can be subjected to intermittent shadow produced by the wind turbine's rotor. The shut-down condition is comprised of two sub-conditions logically combined by an AND: the first sub-condition is that the current position of the sun is such that shadow can theoretically be cast onto the object. The second sub-condition pertains to the “quality and quantity” of the shadow, i.e. whether the sun's light radiation is such that shadow will actually occur. The second sub-condition is based on a measurement of direct- and indirect-light intensities and a result of a comparison of these intensities being above a given threshold.
A similar shadow control is described in U.S. Pat. No. 6,661,111. The shut-down condition is, however, based only on a direct-light measurement, and whether the measured direct-light intensity is above a threshold. When the condition mentioned is fulfilled, the turbine is not shut down instantly, but only after a delay of, e.g., five minutes (such a delay could be regarded as a third sub-condition that is “ANDed” with the other sub-conditions mentioned). If the turbine has been stopped, but the shut-down condition is not fulfilled any more (e.g. because the sun has moved so that no shadow can be cast onto the object anymore, or the measured intensity has fallen below the threshold), the operation of the wind turbine is resumed. Again, operation is not resumed instantly, but only after a delay of, e.g., two minutes. The delays prevent the turbine from being switched on and off too frequently, e.g. if the light intensity fluctuates around the threshold.
US 2006/0267347 A1 pertains to a shadow-control system similar to that of DE 199 28 048 A1. To measure the direct- and indirect-light intensities, three light sensors are provided. They are arranged at relative angles of 120°, and their aperture angle (i.e. their light sensitive opening angle) is at least 120° to ensure that always at least one of the sensors always picks up the direct rays of sunlight and at least another one is not exposed to the direct rays of sunlight.